1. Field of the Invention
This invention relates generally to solid state electronics and more particularly to quantum interference devices.
2. Description of the Prior Art
Three terminal semiconductor devices, including a quantum well between two barriers, are generally known. Such devices implementing both digital and analog circuits result from the need for increased miniaturization, functional density, and operating speed, particularly where nanoelectronic and mesoscopic size regimes are concerned. Such devices are shown and described, for example, in the text entitled Nanostructure Physics And Fabrication, M. A. Reed et al, Academic Press, New York, 1989.
As is well known, a quantum well is comprised of a layered semiconductor structure in which the quantum well layer is sandwiched between two barrier layers of semiconductor or insulator material with larger conduction band energy, larger valence band energy, or both, than the quantum well layer. Electrical carriers (electrons or holes) tunnel resonantly through the barriers and quantum well when the resonant energy states inside the quantum well are favorably aligned with the energy of the carriers outside the barriers. The principle of resonant tunneling through barriers has been described, for example, in a publication entitled, "Resonant Tunneling In Semiconductor Double Barriers" L. L. Chang et al, which appeared in Applied Physics Letters, Volume 24, No. 12, 15 June 1974, pp. 593-595. The resonant energy level inside the quantum well acts as a filter of the electronic wave functions. The energy level of the resonant state within the quantum well places a restriction on the value of momentum in the direction of propagation for the electron wave functions which are transmitted through the quantum well barriers, instead of being reflected. As a result, the only electron wave functions which will be transmitted have a narrow range of values of momentum in the propagation direction. If the electron wave functions are unconstrained in the lateral directions (perpendicular to the direction of propagation), then a subband of transmitted electron energies results. This subband is due to the restriction on momentum in the propagation direction with no restriction placed on the lateral momenta. If the resonant energy level in the quantum well is below the conduction band edge (or above the valence band edge) of the source of carriers outside the barriers, no current can flow through the quantum well. The resonant tunnel diode uses a voltage difference placed across the entire resonant tunneling structure (the voltage difference being applied to semiconductor layers on either side of the barrier layers), to control the resonant energy level inside the quantum well. Constructing a quantum well with a resonant energy level above the conduction band edge, and then using a potential difference to pull it below the band edge results in switching the current from a condition of conduction to a non-conducting condition. Several attempts have been made to control the quantum well resonant energy level independently of the voltages on the layers outside the barriers, both by directly connecting to the quantum well layer and by indirectly influencing the energy level (for example by a quantum Stark effect). These devices have a third electrode controlling the quantum well resonant energy level, so they are resonant tunneling transistors, rather than diodes. These devices are discussed in F. Beltram et al, Applied Physical Letters, Vol. 53, pg 219 (1988); A. A. Grinberg et al, Journal of Applied Physics, Vol. 66, pg 425 (1989); A. C. Seabaugh et al, IEEE Trans. Electronic Devices, Vol. 36, pg. 2328 (1989); and C. H. Yang, Applied Physical Letters, Vol. 60, pg 1250 (1992). However, limited success has been achieved in manufacturing such devices because of the great difficulty in making electrical contact to the quantum well or in indirectly influencing the resonant level without affecting the outside semiconductor layers. Moreover, the resonant tunneling diodes and three terminal transistors described above are large, macroscopic devices, with lateral sizes of 1 micron or larger.
A second category of devices has been proposed for electronic switching in circuits consisting of conductors with extremely small dimensions (conductor widths of less than 100 nanometers). These conductors are often called electron waveguides or quantum wires and are distinguished by widths of less that the coherence lengths of the electron wave function (the so called "mesoscopic dimension"). The proposed switching devices operate on a quantum interference principle, and as a result all their critical dimensions must be mesoscopic. These quantum interference devices are discussed in the text by Reed et al. These devices have a major problem in their sensitivity to their environment, in particular, to thermal fluctuations. This sensitivity is due to the requirement for electron wave functions to travel by two separate paths and to recombine forming an interference pattern. This kind of interference is similar to the interference in a Michelson Interferometer, which is known to be very sensitive to thermal and other environmental conditions. Fluctuations of a random nature in either or both paths will destroy the interference pattern and will nullify the switching mechanism of the device.